1. Field of the Invention
The present invention relates to an electronic part-incorporated multilayer substrate and a method of producing the same. More particularly, the invention relates to an electronic part-incorporated multilayer substrate having a structure in which a core made of a metal or an organic material and formed with an electronic part accommodation part is sealed with an insulating resin layer. The invention also relates to a method of producing such a multi-layer substrate.
2. Description of the Related Art
As a result of the diversification of portable equipment, an electronic part-incorporated multilayer substrate having a structure as described above have come into use as functional modules, semiconductor packages, and mother boards for various applications including cellular phone, portable electronic dictionaries, personal information instruments (sometime abbreviated as PDAs), and digital cameras (sometimes abbreviated as DSCs). Specifically, applications of such multi-layer substrates include:
1) parts-incorporated module substrate, e.g., radio modules, power supply modules, camera modules
2) parts-incorporated semiconductor package substrate, e.g., capacitor-incorporated substrate for decoupling
3) small and thin mother boards with built-in parts
Such electronic part-incorporated multilayer substrate may be fabricated using various methods depending on the scale of electronic circuits to be incorporated therein. In particular, a substrate of this type may be designed using a build-up industrial method to provide the substrate with required layers. The substrate may be designed by categorizing chip-type passive parts such as capacitors, resistors, and inductors and active parts such as semiconductor modules into parts to be incorporated and parts to be externally mounted. Various methods and configurations are used to package electronic part-incorporated multilayer substrate with high density and high accuracy. Recently, there are demands for electronic part-incorporated multilayer substrate having smaller thicknesses.
Known techniques for satisfying such demands in the related art include, for example, the techniques disclosed in Patent Documents 1 and 2 listed below.
Patent Document 1 discloses an electronic part-incorporated multilayer substrate, which has a structure similar to that of a multi-layer substrate according to the related art. The multi-layer substrate includes a conductor pattern formed on an insulating substrate to serve as a core. Active parts are mounted on the conductor pattern. A dam formation material is applied to a height in accordance with the height of the active parts mounted as described above. A cavity for accommodating electronic parts is defined by the dam formation material. The cavity is filled with an insulating resin such that electronic parts are surrounded by the resin. The document discloses a structure and a production method for building up further layers accommodating electronic parts in the same way as occasion demands.
There are demands for more significant reductions in the thickness of multi-layer substrates than that achievable with the structure formed by a multi-layer substrate carrying electronic parts and a dam formation material proposed in Patent Document 1. Under the circumstance, proposals have been made to introduce multi-layer substrates in which an insulating substrate serving as a core as proposed in Patent Document 1 is replaced with a thin resin layer and in which a substrate formed with through holes for accommodating electronic parts is used as a core. Such a multi-layer substrate is disclosed in Patent Document 2 and will be described in detail below.
A multi-layer substrate according to Patent Document 2 has a core having through holes, and a sheet having an insulating resin layer formed thereon is applied to one side of the core. Thus, the through holes are closed at one side thereof, and holes to serve cavities are formed. Electronic parts are disposed in the holes and are secured on the insulating resin layer using an adhesive. The holes are filled with an insulating resin such that the electronic parts are surrounded by the resin. A wiring layer having a wiring pattern is formed on an outer surface of the insulating resin layer.
FIG. 7 and FIG. 8 are illustrations showing some of steps of producing an electronic part-incorporated multilayer substrate according to Patent Document 2. First, as shown in FIG. 7A, a half callousness resin sheet 103, which may be referred to as “B-stage resin”, laminated with copper foil 102 is bonded to one side of a metal core 101 formed with a part accommodation part 100 in the form of a through hole. The resin sheet 103 is made of a material having insulating properties, and an insulating resin layer is therefore formed by the resin sheet 103.
Next, as shown in FIG. 7B, an appropriate amount of liquid adhesive 104 is supplied to the part accommodation part 100 by a dispenser. Thereafter, an electronic part 105 such as an active part or a passive part is placed in the part accommodation part 100 as shown in FIG. 7C, and the liquid adhesive 104 is then cured to secure the electronic part 105. Next, as shown in FIG. 8A, a half callousness resin sheet 107 laminated with copper foil 106 is disposed to cover the electronic part 105 in the part accommodation part 100 and the cured liquid adhesive 104. The resin sheet 107 is heated to fill in the part accommodation part 100 with the resin as shown in FIG. 8B. An insulating resin layer is formed by the resin sheet 107. A filler made of an inorganic material having insulating properties is included in the resin sheet 107 such that the linear expansion coefficient of the sheet is kept small enough to prevent thermal stress attributable to thermal expansion from being exerted on the electronic part.    Patent Document 1: JP-A-2003-031954    Patent Document 2: JP-A-2005-311249
The above-described technique in the related art has the following problems.
First, the insulating resin layer 103 and the insulating resin layer 107 must have predetermined thicknesses to keep the metal core 101 and the electronic part 105 insulated from the wiring layer that is formed later. Since the resin sheet 103 and the resin sheet 107 to serve as insulating resin layers are half callousness, there is a problem in that the thicknesses of the layers can vary because of deformation of the same. It is therefore difficult to obtain layer thicknesses which are sufficient to maintain desired insulating properties. Further, there is a difficulty in reducing the thickness of the substrate having the layers.
Second, the resin sheet 107 includes a filler for keeping the linear expansion coefficient of the sheet small enough. When the amount of the filler is great, a problem arises in that there is no room for increasing the amount of the filler to prevent any reduction in adhesive power between the resin layer 107 and a wiring conductor which is formed on the resin layer 107 after the copper foil 106 is removed.
Third, since the electronic part 105 is secured by supplying the liquid adhesive 104 in a required amount to the part storage part 100, the height of the electronic part 105 can vary depending on the amount of the liquid adhesive 104 supplied. For example, when the amount supplied is too great, the electronic part 105 is raised by the liquid adhesive 104 supplied in an excessive amount. A difficulty in adjustment therefore arises because the height of the electronic part 105 is increased accordingly, and management must be carried out with higher accuracy, which can result in a cost increase.
Fourth, the work of supplying an appropriate amount of the liquid adhesive 104 to a number of the part storage parts 100 by means of a dispenser or the like necessitates supplying under one-by-one control regardless of work by human or machine, which becomes a factor causing an increase in cost.